1. Technical Field of the Invention
The present invention relates to electrochromic devices for continuously varying the transmissivity to light suitable for use in, for example, electrochromic rearview mirrors, windows and sun roofs for motor vehicles, manufactured from electrochromic solid films and electrolytes containing redox reaction promoters and alkali ions and/or protons.
2. Brief Description of the Related Technology
Prior to the introduction of electro-optic mirrors into the automotive marketplace, prismatic rearview mirrors were available to drivers of motor vehicles to determine the whereabouts of neighboring motor vehicles to their rearward surroundings. By using a manual lever located on such mirrors, a driver of a motor vehicle, especially at dusk or later, would be able to employ a prismatic feature on the mirror to vitiate the effect of headlamp glare (the principal source of incoming electromagnetic radiation from the rear of the motor vehicle) from the low beam, and especially high beam, lighting elements of other motor vehicles travelling posterior thereto. Should the lever be flipped to the nighttime position, the driver would be able to view an image in a reflection from a glass-to-air interface on the first surface of the mirror. The light reflected from this first surface would exhibit non-spectral selectivity. That is, the background of any image viewed in the nighttime position of the prismatic mirror would be a neutral color. Such conventional prismatic mirrors are still used on a majority of motor vehicles in the United States today.
With the advent of electro-optic technology, such as electrochromic technology, it has become possible to achieve continuous variability in reflectivity in rearview mirrors for motor vehicles. This continuous variability has been achieved, for example, through the use of reversibly variable electrochromic devices, wherein the intensity of light (e.g., visible, infrared, ultraviolet or other distinct or overlapping electromagnetic radiation) is modulated by passing the light through an electrochromic medium. In such devices, the electrochromic medium is disposed between two conductive electrodes and undergoes electrochromism when potential differences are applied across the two electrodes.
Some examples of these prior art electrochromic devices are described in U.S. Pat. Nos. 3,280,701 (Donnelly); 3,451,741 (Manos); 3,806,229 (Schoot); 4,465,339 (Baucke); 4,712,879 (Lynam) ("Lynam I"); 4,902,108 (Byker) ("Byker I"); Japanese Patent Publication JP 57-30,639 (Negishi) ("Negishi I"); Japanese Patent Publication JP 57-208,530 (Negishi) ("Negishi II"); and I. F. Chang, "Electrochromic and Electrochemichromic Materials and Phenomena", in Nonemissive Electrooptic Displays, 155-96, A. R. Kmetz and F. K. von Willisen, eds., Plenum Press, New York (1976).
Numerous devices using an electrochromic medium wherein the electrochromism takes place entirely in a liquid solution are known in the art [see e.g., U.S. Pat. Nos. 5,128,799 (Byker) ("Byker II"); Donnelly, Manos, Schoot and Byker I; and commonly assigned U.S. Pat. Nos. 5,073,012 (Lynam) ("Lynam II"); 5,115,346 (Lynam) ("Lynam III"); 5,140,455 (Varaprasad) ("Varaprasad I"); 5,142,407 (Varaprasad) ("Varaprasad II"); 5,151,816 (Varaprasad) ("Varaprasad III"); 5,239,405 (Varaprasad) ("Varaprasad IV"); and commonly assigned co-pending U.S. patent application Ser. Nos. 07/935,784 (filed Aug. 27, 1992), now U.S. Pat. No. 5,500,760, and 08/061,742 (filed May 17, 1993), now U.S. Pat. No. 5,424,865]. Typically, these electrochromic devices, sometimes referred to as electrochemichromic devices, are single-compartment, self-erasing, solution-phase electrochromic devices. See e.g., Manos, Negishi II, Byker I and Byker II.
In single-compartment, self-erasing, solution-phase electrochromic devices, the intensity of the electromagnetic radiation is modulated by passing through a solution of the color-forming species held in a single-compartment. The color-changing reaction occurs only in this solution-phase. That is, there is no solid material present in the devices that has the color-changing reaction in it. During operation of such devices, the solution of the color-forming species is liquid or fluid, although it may be gelled or made highly viscous with a thickening agent, and the components of the solution do not precipitate. See e.g., Byker I and Byker II.
Numerous devices using an electrochromic medium wherein the electrochromism occurs in a solid layer are also widely described in the art. Among such devices are those that employ electrochromic thin film technology [see e.g., N. R. Lynam, "Electrochromic Automotive Day/Night Mirrors", SAE Technical Paper Series, 870636 (1987); N. R. Lynam, "Smart Windows for Automobiles", SAE Technical Paper Series, 900419 (1990); N. R. Lynam and A. Agrawal, "Automotive Applications of Chromogenic Materials", Large Area Chromogenics: Materials & Devices for Transmittance Control, C. M. Lampert and C. G. Granquist, eds., Optical Eng'g Press, Washington (1990); C. M. Lampert, "Electrochromic Devices and Devices for Energy Efficient Windows", Solar Energy Materials, 11, 1-27 (1984); Japanese Patent Document JP 58-30,729 (Kamimori) ("Kamimori I"); U.S. Pat. Nos. 3,521,941 (Deb); 3,807,832 (Castellion); 4,174,152 (Giglia); Re. 30,835 (Giglia); 4,338,000 (Kamimori) ("Kamimori II"); 4,652,090 (Uchikawa); 4,671,619 (Kamimori) ("Kamimori III"); 4,702,566 (Tukude); Lynam I and commonly assigned U.S. Pat. Nos. 5,066,112 (Lynam) ("Lynam IV") and 5,076,674 (Lynam) ("Lynam V")].
In thin film electrochromic devices, an anodic electrochromic layer and/or a cathodic electrochromic layer, each layer usually made from inorganic metal oxides or polymer films, may be separate and distinct from one another. In contrast to the single-compartment, self-erasing, solution-phase devices referred to supra, these thin film electrochromic devices modulate the intensity of electromagnetic radiation by passing through the individual anodic electrochromic layer and/or cathodic electrochromic layer.
In certain thin film electrochromic devices, a thin film layer of a solid electrochromic material, such as a tungsten oxide-type solid film, may be placed in contact with a liquid electrolyte containing redox promoters, such as ferrocene and iodide, and a solvent. See e.g., Kamimori III. In these electrochromic devices, the intensity of electromagnetic radiation is primarily modulated by passing through the solid electrochromic material. When dimmed to a colored state, these tungsten oxide-type solid films typically dim to a blue-colored state.
Having grown accustomed to conventional prismatic rearview mirrors for motor vehicles, some consumers of motor vehicles may show a preference for rearview mirrors possessing substantial non-spectral selectivity. That is, some consumers may prefer mirrors which present a substantially gray color when dimmed to a colored state; in other words, a mirror that exhibits a viewing background comparable in spectral reflectivity to that of conventional prismatic mirrors.
On another note, the reflective element of the mirror is often constructed from silver and is typically situated on the rearmost surface of the mirror. That is, the reflective element is placed on the surface of a glass substrate farthest from that surface which first comes in contact with incident light. However, such placement has certain disadvantages. For instance, double imaging is a recognized problem in such mirror construction. In addition, in its path to reaching the reflective element of the mirror, incident light must first pass through each of the glass substrates of the mirror assembly. Therefore, in these mirror constructions, to achieve good optical performance, higher quality glass should be used for both substrates. Moreover, these mirror constructions typically require the use of a thin film transparent conductive electrode coating on the inward surface of each substrate in order to apply a potential to the electrochromic element. Requiring each substrate of the mirror to be of such higher quality glass and the use of two such transparent conductive electrodes increases material and production costs. Further, placement of the reflective element on the rearmost surface of the mirror requires an additional manufacturing step, which also increases production costs. And, such placement increases material and production costs due to necessary measures taken to protect the reflective element (typically, a highly reflective material, such as silver or aluminum) against environmental degradation, such as through the use of a paint or the like. Frequently, lead-based paints have been used for this purpose, thereby presenting environmental concerns.
It has been suggested and attempts have been made to place the reflective element of the mirror, such as silver, on the inward facing surface of the rear substrate so as to act as a conductive electrode coating as well as a reflective element. See e.g., Donnelly, Negishi I, Byker I and Byker II. This configuration is plainly attractive since it eliminates the need for a separate transparent conductive coating on the rear substrate, thereby reducing the cost of manufacture.
In order to function in the dual role of reflective element and conductive electrode, a coating must (1) be electrochemically stable so as not to degrade during operation of the device, (2) remain securely adhered to the rear substrate to maintain the integrity of the device, and (3) be highly reflective so that the mirror as a whole will have an acceptable level of reflectance. However, no known mirror construction meets all of these requirements--for example silver, commonly used as the reflective element in conventional mirror constructions, is highly reflective but is not electrochemically stable and is difficult to adhere to the surface of a glass substrate. Other materials, such as rhodium or Inconel, which have been used as a combined reflective element and conducting electrode in prior art mirrors are not sufficiently reflective to provide a highly reflective electrochromic mirror. Perhaps for these reasons, the prior art suggestions and attempts have not resulted in any commercially successful electrochromic mirror in which a single coating is used as both reflective element and conducting electrode.
Electrochromic devices, such as those using a solid film electrochromic material, like tungsten oxide, may also exhibit deleterious performance when exposed to ultraviolet radiation over prolonged periods of time (e.g., conditions typically encountered during outdoor weathering). This deleterious performance may be linked to any of a variety of sources, including a potential propensity for photochromism to occur.
On yet another note, displays, indicia and sensors, such as photosensors, motion sensors, cameras and the like, have heretofore been incorporated into certain electrochromic mirror constructions [see e.g., U.S. Pat. Nos. 5,189,537 (O'Farrell) and 5,285,060 (Larson)]. In these constructions, the reflective element of the mirror has been locally removed to create a highly transmissive local window. However, such use of displays and the like positioned behind the reflective element of electrochromic mirrors has been limited. One reason for this limited use is due to diminished rear vision capability in that portion of the reflective element of the mirror which has been removed. Moreover, the displays and the like known to date may be distracting as well as aesthetically non-appealing to the driver and/or passengers of motor vehicles insofar as they may be visible and observable within the mirror mounted in the motor vehicles when in the inactivated state. In addition, the known methods of incorporating such displays and the like into mirrors have been only partially successful, labor intensive and economically unattractive from a manufacturing standpoint.
Further, although it has been suggested to use semi-transparent reflectors in rearview mirrors [see e.g., U.S. Pat. Nos. 5,014,167 (Roberts) ("Roberts I") and 5,207,492 (Roberts) ("Roberts II")], previous attempts have included the use of dichroic reflectors which are complex to design and expensive to fabricate. Also, where use of metallic reflectors has been suggested [see e.g., U.S. Pat. No. 4,588,267 (Pastore)], it has been in the context of conventional mirrors such as prismatic mirrors. These suggestions fail to recognize the problems that must be overcome to provide a highly reflecting and partially transmitting electrochromic rearview mirror.
Therefore, the need exists for an electrochromic mirror that provides substantial non-spectral selectivity when dimmed to a colored state, akin to that exhibited by conventional prismatic mirrors when in the nighttime position, along with continuous variability in reflectivity, ease and economy of manufacture and enhanced outdoor weathering resilience. It would also be desirable, particularly in this connection, to have an electrochromic mirror construction that reduces material and manufacturing costs by employing as only one of its substrates a high quality glass as a substrate and also as only one of its electrodes a thin film, substantially transparent conductive electrode coating. In addition, it would be desirable for a mirror to have display-on-demand capability where a display could become activated to be viewed on demand, and where the display is (1) aesthetically appealing and not distracting in its inactivated state, and (2) is manufactured with ease and economy.